The invention relates to two cycle crankcase compression fuel injected internal combustion engines, and more particularly to accurate control of the fuel injection.
In a two cycle fuel injected internal combustion engine, the flow of combustion air into the crankcase of the engine is controlled by one or more throttle valves. A throttle position sensor, far example as shown in U.S. Pat. No. 4,280,465, incorporated herein by reference, senses rotation of the throttle valve shaft and provides such information to control circuitry which determines fuel injector pulse width. For accurate computation of the pulse width, good resolution of the throttle position is needed. This is difficult at low speed because small changes in throttle opening cause large changes in power and speed. In contrast, at higher speeds, a larger increase in throttle opening is needed to cause small changes in power and speed. There is a need for better resolution at low speed small throttle openings.
The present invention addresses and solves the above need. In an intake manifold having first and second sets of one or more throttle valves, for example as shown in U.S. Pat. No. 4,702,202, incorporated herein by reference, a progressive throttle linkage is provided in accordance with the present invention which is movable to open the first set of throttle valves through a given initial range of motion prior to opening the second set of throttle valves. In the preferred embodiment, the first set of throttle valves rotate through 50% of their motion before the second set of throttle valves begin to open. Both sets of throttle valves reach wide open throttle position substantially simultaneously. The throttle position sensor is coupled to the pivot shaft for the first set of throttle plates and controls fuel injection according to throttle position, to provide increased resolution of sensed throttle position at low engine speed because combustion air is flowing only through the first set of throttle valves and not through the second set of throttle valves, whereby greater movement of the first set of throttle valves is required to obtain a given amount of combustion air flow, prior to opening the second set of throttle valves. This provides more accurate fuel injection. The throttle position sensor is a potentiometer that has a linear scale over its entire 75.degree. of throttle shaft rotation. Opening one set of throttle valves instead of two provides more throttle shaft rotation and hence greater throttle position sensor resolution, for a given engine speed. This also provides smoother throttle response and control by the operator.